The extreme miniaturization of sound amplification systems such as found in headworn hearing aids is achieved by incorporating, miniaturized components into compact, wearable system assemblies. Such a system usually comprises a microphone, two or more stages of amplification, a gain control, a battery power source, and a sound reproducer, called a receiver. These components, when packaged in a tiny, appropriately shaped housing, constitute a minimum hearing aid. To achieve hearing aids that are cosmetically unobtrusive, each of these components has been miniaturized as much as the economics and state of the art have permitted.
One of the practical problems in designing such a miniaturized system results from interactions within the operating circuits. To produce amplified sound in the user's ear, the amplifier must deliver signal-frequency electrical power to the receiver. The amplifier obtains this energy from a tiny battery incorporated in the device. Because the battery is very small, its internal impedance as a voltage source is not negligible. The draining of energy from the battery causes the power supply voltage to fluctuate in a manner dependent on the amplified signal. In addition to supplying the energy to operate the amplifier stage which powers the receiver, the battery also serves as a power supply for a preamplifier which strengthens the weak signal from the microphone so that it can activate the power amplifier. These battery voltage fluctuations, when applied to the preamplifier where the signals are very weak, intermix with the desired microphone signal to such a degree as to produce distortion, system instability, or both.
A conventional approach to this problem is to introduce an R-C filter circuit, comprising a series resistance and a shunt capacitance, between the battery and the preamplifier. This has two drawbacks. First, it is necessary to provide additional space, in some part of the system, to accommodate the filter resistor and capacitor. The filter capacitor typically has a value of one to ten microfarads. The spaced required for such a capacitor increases the size of the hearing aid significantly. Second, the battery voltage, in addition to fluctuating with the signal, may also decrease in average value depending on the average rate of energy consumption during the life of the battery. This causes a decrease in amplification available from the preamplifier stage and, therefore, an unpredictable amplification from the complete system.
Microphones used in modern hearing aids traditionally incorporate the preamplifier within the microphone housing. Such microphones are described in U.S. Pat. Nos. 3,816,671 and 4,063,050. In some instances, if the housing is large enough, a filter capacitor and resistor may be included inside the microphone housing. However, this arrangement is likely to result in some degradation in electro-acoustic performance of the microphone. Moreover, in the smaller modern microphones the housing is not large enough to provide adequate space for such a filter capacitor within the housing.
In another approach to this problem, the integrated circuits used as amplifiers have been equipped with voltage regulating circuits. This solves the problem of varying amplifier gain, but the circuits currently available require the use of capacitors of substantially the same size as used with the previously described resistor/capacitor filter networks in order to achieve reasonable electrical stability. Thus, this alternative also has substantial space problems.
The batteries used to power the amplifiers in these miniature transducer systems produce voltages between 1.25 and 1.55 volts. Any filter or voltage regulator circuit must not reduce this voltage below a level at which satisfactory amplifier operation can be reliably obtained. With present day techniques, a voltage of 0.9 to 1.0 volts is usually adequate.